Epoxide-cured acrylo-butadiene copolymers

ABSTRACT

1. AS A NEW COMPOSITION OF MATTER, THE ELASTOMERIC REACTION PRODUCT OF A POLYEPOXIDE AND AN ACRYLO-BUTADIENE COPOLYMER HAVING FREE CARBOXYL GROUPS. 2. THE ELASTOMERIC REACTION PRODUCT OF A POLYEPOXIDE AND A COPOLYMER OF BUTADIENE AND ACRYLIC ACID. 6. THE METHOD OF MAKING AN ELASTOMER WHICH COMPRISES COPOLYMERIZING A DIENE SELECTED FROM THE GROUP CONSISTING OF BUTADIENE AND LOWER ALKYL SUBSTITUTED BUTADIENES WITH A ACRYLIC COMPOUND SELECTED FROM THE GROUP CONSISTING OF ACRYLIC ACID, LOWER ALKYL SUBSTITUTED ACRYLIC ACIDS AND THE ESTERS OF SAID ACIDS TO FORM A LIQUID COPOLYMER WITH A LIQUID POLYEPOXIDE, AND HEATING THE MIXTURE TO CAUSE SAID POLYEPOXIDE TO REACT WITH SAID COPOLYMER TO FORM A RUBBER-LIKE SOLID.

United States Patent l 3,563,966 EPOXIDE-CURED ACRYLO-BUTADIENE COPOLYMERS Robert Dean Lowrey, Hopkins, Minn, and William Edward Hunter, Huntsville, Ala., assignors to Thiokol Chemical Corporation, Trenton, N..l., a corporation of Delaware N0 Drawing. Continuation-in-part of application Ser. No. 644,954, Mar. 8, 1957. This application Jan. 6, 1958, Ser. No. 707,444

Int. Cl. C081? 15/00, 15/40 US. Cl. 260--82.1 14 Claims This application is a continuation-in-part of our copending application Ser. No. 644,954 filed Mar. 8, 1957, now abandoned.

This invention relates to combustible compositions that contain the oxygen required for their combustion, and more particularly to a novel solid composition of this type that comprises a predominantly hydrocarbon fuel and an oxidizing agent therefor, as well as to a method of making such compositions. Compositions of this type have been used to produce the necessary high temperature gases required for the propulsion of rockets, aircraft boosters, missiles and the like, and for convenience will be referred to herein as propellants, although as the description proceeds it will become apparent that the compositions can also be employed for certain nonpropulsive uses e.g. gas or smoke generators.

It has been recognized for many years that hydrocarbons because of their relatively high heat of combustion per unit weight are desirable fuels for rocket propulsion. The hydrocarbons have been used with a degree of success as liquid fuels. However, liquid fuels require more or less elaborate pumping systems to pump them to the point where they are to be burned and mixing devices for mixing them with a material containing the oxygen for their combustion.

The need for pumps and mixing devices can be circum vented by employing a solid propellent, but difliculties have been encountered in formulating a solid propellent capable of satisfying the numerous practical requirements that must be met. Thus the propellant should desirably be castable i.e. readily convertible by curing or otherwise from a liquid to a solid state in situ in a container. More particularly the propellant should be capable of existing in a sufficiently fluid state to permit it to be poured or extruded into a metal container having an intricate internal configuration without limitation as to size or shape, and converted in situ therein into a solid mass. Moreover, the conversion from the liquid to the solid state in situ in the metal container should occur without evolution of gas or significant dimensional changes in the mass, so that a charge of uniform and reproducible density will be obtained.

The propellant charge must be capable of burning completely at a predictable rate. If the combustion is irregular or incomplete due to non-homogeneity of the propellant, erratic performance of the rocket motor, or other device in which the propellant is used, will result. If the propellant is of a brittle or friable character, it may disintegrate to some extent due to physical or thermal shocks to which it may be subjected, thereby producing cracks or fissures that greatly increase the exposed surface area. Since the combustion normally occurs at exposed surfaces of the propellant, the formation of such cracks greatly increases the rate of gas evolution and in some cases may produce a pressure increase sufficent to rupture the motor casing. Cracks in the propellant may also cause premature burn-through of the propellant web, and thereby permit local overheating of the case and consequent rupture. It is further necessary that the propellant charge remain in satisfactory firing condition when Patented Feb. 16, 1971 it is subjected to relatively low atmospheric temperatures. Preferably the charge should retain its elasticity and resistance to shock at temperatures as low as 70 F.

Though many plastic materials are liquids which can be mixed and handled easily and thereafter converted to a solid form, their chief deficiency for use as a fuelbinder in this type of propellant arises from the fact that after curing nearly all of the readily available plastics lack flexibility and resilience, and thereby give poor performance at low temperatures (below 32 F A notable exception, plasticized polyvinyl chloride, though flexible, is less desirable than an essentially hydrocarbon polymer because of its high percentage of chlorine. The most successful propellants of this general type have employed a mixture of oxidizing agent and liquid polysulfide polymer which is cured in situ in a metal casing to provide a propellant grain or charge that remains shock resistant at relatively low temperatures. While this type of propellant more nearly meets the many requirements mentioned above than any other previously proposed composition, it is still subject to a number of disadvantages. Thus the presence of the sulfur reduces the specific impulse of the composition, i.e. the total useful energy developed per unit weight of material.

It is accordingly an object of the present invention to provide a solid combustible composition that has improved properties when used as a missile propellant. It is another object of the invention to provide an improved binder and fuel for such a propellant composition. It is still another object of the invention to provide solid propellant composition that is made from relatively inexpensive raw materials and has acceptable low temperature properties. It is a still further object of the invention to provide an organic binder and fuel for such a propellant that burns without leaving a solid residue and can be cured with a curing agent that burns without leaving a solid residue. It is still another object of the invention to provide a binder and fuel material of this type that can be cured by a curing agent without evolution of gas and without substantial dimensional changes in the mass. It is still another object of the invention to introduce functional groups into essentially hydrocarbon polymers intended for formulation into castable propellants to render them susceptible to curing to a solid combustible composition that has properties useful as a propellant. Other objects of the invention will be in part obvious and in part pointed out hereafter.

The objects of the invention can be achieved in general by utilizing as the binder-fuel of a solid propellant composition an epoxide-cured acrylo-butadiene copolymer. In carrying out a preferred embodiment of the invention an acrylo-butadiene copolymer having carboxyl groups is first prepared. Such a polymer may be prepared in various ways. For example, butadiene: or a substituted butadiene, e. g. a lower alkyl substituted butadiene such as isoprene or dimethylbutadiene may be copolymerized with acrylic acid or a substituted acrylic acid such as methacrylic acid to yield a polymer containing a carboxyl group or groups. Alternatively the substituted or unsubstituted butadicne can be copolymerized with an ester of acrylic acid or of a substituted acrylic acid, e.g. a lower alkyl acrylate, and the resulting copolymer hydrolyzed to eliminate the alkyl groups. Such copolymers are referred to herein as acrylo-butadiene copolymers, and details concerning their preparation are given in the specific examples below.

The liquid copolymer as thus prepared is mixed with a liquid polyepoxide curing agent and a suitable oxidizing agent. Inorganic non-metallic oxidizing agents such as ammonium perchlorate and ammonium nitrate are preferred. The oxidizing agent ordinarily comprises from 3 to 7 parts per part by weight of the binder-fuel. The polyepoxide may be any of the known liquid polyepoxide compositions. It may be used in the proportions of 0.1 to 0.33 part by weight per part of copolymer.

If the propellant mixture as thus prepared is to be used in making a rocket motor, it is then introduced into the interior of a motor casing. An illustrative propellant loading procedure is given in the examples. The propellant mixture is cured in situ in the casing by heating to an elevated temperature for a period of time, e.g. 170 F. for 24 hours, during which period the propellant mixture is converted to a solid mass. Solid propellant motors as thus made have excellent shock resistant characteristics and retain such characteristics at relatively low temperatures.

In order to point out more fully the nature of the present invention, the following specific examples are given of methods of making the copolymers and propellant compositions of the present invention.

EXAMPLE 1 A mixture of butadiene and methyl acrylate are copolymerized in aqueous emulsion in the following manner. A polymerization reaction vessel provided with an agitator and capable of being sealed is charged with 43.2 parts by weight of 1,3-butadiene, 7.64 parts of methyl acrylate, 100 parts of water that has previously been freed from air by boiling or distillation, 1 part of dioctyl sodium sulfosuccinate emulsifying agent, 8 parts of dodecyl mercaptan as a modifier, and 0.18 part of potassium persulfate as an initiator. The reaction vessel is sealed shut and the mixture therein agitated at a suitable temperature within the range of 40-60 C., e.g. 47 C., for a number of hours. The progress of the reaction is followed by periodically removing samples and analyzing for total solids with a correction being made for the emulsifier. When the total solids in an aliquot reaches 70% by Weight of the total weight of monomers charged, the reaction is considered complete. This point is sometimes called 70% conversion and may be reached in to 52 hours depending on the batch size, composition and various other factors.

When samples taken show 70% conversion, the emulsion is run into approximately parts by weight of a solution of potassium aluminum sulfate containing 10% by weight of the alum calculated with the water of hydration included. The resulting mixture is warmed to remove unreacted butadiene which can be recovered and reused. The copolymer creams to the top and is separated from the aqueous layer which is discarded.

The acrylo-butadiene copolymer is then hydrolyzed with 15% by weight aqueous sodium hydroxide to replace the methyl groups thereof by sodium. The amount of sodium hydroxide solution used is 100% in excess of that required on the basis of stoichiometrical calculations. The mixture of polymer and caustic soda is stirred and heated at 80100 C. until hydrolysis is complete, the time required being usually about 6 hours or more. The progress of the hydrolysis reaction may be followed by taking samples from time to time; neutralizing, washing and drying the samples; and determining their infra-red absorption characteristics. Hydrolysis is considered complete when the infra-red absorption shifts from 1735 cm. to 1700 cm. i.e. from the characteristic absorption of an ester to the characteristic absorption of a carboxyl group.

The hydrolysis reaction converts the product to the sodium salt of a butadiene-acrylic acid copolymer. When hydrolysis is complete, spent alkali is separated and dilute hydrochloric acid is added to the polymer until it is neutral to litmus paper to form the free acid. The polymer is then separated, thinned with dioxane, and separated by dilution with water. The polymer is washed until it is new tral, separated from the wash water, and dried under vacuum in a steam-heated falling-film evaporator of convenuonal design. Complete removal of water is evidenced by the fact that the polymer is clear and no longer cloudy.

The polymer as thus made is incorporated in a propellant mix. Thus 13.06 parts by weight of the liquid polymer are mixed with 2.77 parts by weight of a commercial liquid polyepoxide curing agent sold under the trade designation BR 18795. This product is understood to be an epoxide curing agent made from bis-phenol A and epichlorhydrin and having the structure Me C (C H OCH CHCH O para) 2 or a low multiple thereof. The commercial product is believed to be largely monomer with minor amounts of dimer and trimer.

The liquid polymer and the epoxide are introduced into a sigma blade Baker-Perkins mixer and mixed at low speed, e.g. 33 r.p.m. Thereafter 84.17 parts of ammonium perchlorate are added. The ammonium perchlorate preferably comprises a mixture of two grain sizes, say by weight having a particle size such that grains will be retained by a mesh screen and 25% having a particle size of about 5 to 80 microns. The time required for mixing varies with the size of the batch; for a 4000 gram batch, 45 minutes is required.

The propellant mixture as thus prepared is introduced into a rocket motor and cured to a solid mass in situ therein by being heated in a curing oven for a suitable period, e.g. 24 hours at F. The propellant is usually cast around a central mandrel which is later withdrawn to provide an open space having any of various crosssectional configurations and extending through the propellant charge. A suitable electrically-actuatable igniter is introduced into the motor casing for ignition of the charge.

It has been found that propellants of the type described above have a burning rate of 0.336 in./sec. at 1000 p.s.i. and a specific impulse of 240 lb.-sec./lb.

EXAMPLE 2 Acrylo-butadiene copolymers of the type referred to herein can also be made by reaction between butadiene and acrylic acid as illustrated in the present example. A polymerization reaction vessel is charged with 87.1 parts by weight of butadiene, 12.9 parts of acrylic acid, parts of water which has been previously boiled or distilled to remove air, 17.92 parts of a 25% aqueous solution of benzylcetyldimethylammonium chloride as an emulsifying agent, 14.93 grams of dodecylmarcapta-n as a modifier, and 0.265 to 0.35 part of 2,2 azo bis isobutyronitrile as an initiator. The butadiene and acrylic acid are freed from stabilizers before being charged to the reaction vessel.

After charging, the reaction vessel is sealed and the mixture therein is agitated at a suitable temperature within the range of 4060 C., e.g. 47 C., until the conversion (as defined in Example 1) reaches 70-76%.- The batch is then run into 25 parts of 15% by weight aqueous sodrum chloride and warmed to expel unreacted butadiene, which may be recovered if desired. The polymer creams to the top and is washed until a sample of the filtered washings no longer gives a test for chloride ion with silver nitrate solution. The product is then de-Watered in an evaporator as described in Example 1.

The product as thus prepared can be mixed with a polyepoxide and oxidizing agent as described in Example 1 to provide a readily curable propellant mixture.

EXAMPLE 3 A polymerization reaction vessel is charged with 92.3 parts by weight of butadiene, 7.7 parts of methacrylic acid, 17.2 parts of a 25 aqueous solution of benzylcetyldimethylammonium chloride as an emulsifying agent, 180 parts of water which has been previously boiled or distilled to remove air, 14.34 parts of n-dodecyl mercaptan as a modifier, and 0.2 part of 2,2 azo-bis isobutyronitrile as an initiator.

After charging, the reaction vessel is sealed and the mixture therein is agitated at a suitable temperature within the range of 40-60 C., e.g. 47 C. until the conversion (as defined in Example 1) reaches 50% to 70%. The batch is then run into 25 parts of by weight aqueous sodium chloride and warmed to expel unreacted butadiene, which may be recovered if desired. The liquid polymer creams to the top and is washed with water until a sample of the filtered washing no longer gives a test for chloride ion with silver nitrate solution. The polymer is then tie-watered in an evaporator as described in Example 1.

The polymer as thus prepared can be incorporated in a propellant as described in Example 1.

EXAMPLE 4 A polymerization reaction vessel is charged with 89.5 parts by weight of isoprene, 10.5 parts of acrylic acid, 180 parts of Water which has been previously boiled or distilled to remove air, 17.2 parts of a aqueous solution of benzylacetyldimethylammonium chloride as an emulsifying agent, 14.34 parts of n-dodecyl mercaptan as a modifier, and 0.3 part of 2,2-azo-bis-isobutyronitrile as an initiator.

After charging, the reaction vessel is sealed and the mixture therein is agitated at a suitable temperature within the range of 40-60 C., e.g. 47 C. until the conversion (as defined in Example 1) reaches 50% to 70%. The batch is then run into 25 parts of 10% by weight aqueous sodium chloride and warmed to expel unreacted isoprene, which may be recovered if desired. The liquid polymer creams to the top and is washed with water until a sample of the filtered washing no longer gives a test for chloride ion with silver nitrate solution. The polymer is then de-watered in an evaporator as described in Example 1.

The polymer as thus prepared can be incorporated in a propellant as described in Example 1.

EXAMPLE 5 The procedure of Example 1 is followed except that 14.85 parts by weight of the liquid polymer, 3.15 parts of the liquid polyepoxide curing agent, and 740 parts of ammonium perchlorate are used. Also 8.0 parts by weight of 3 to 8 micron aluminum powder is added to the mixer after introduction of the liquid polymer and epoxide thereinto and before addition of the ammonium perchlorate.

It has been found that when this propellant mix is introduced into a rocket motor and cured as in Example 1, the propellant has a burning rate of 0.310 in./ sec. at 100 p.s.i. and a specific impulse of 240 lb.-sec./lb. at optimum nozzle expansion at sea-level pressure.

It will, of course, be understood that the foregoing examples are illustrative only and that numerous changes can be made in the materials, proportions, and conditions described without departing from the spirit of the invention as set forth in appended claims.

We claim:

1. As a new composition of matter, the elastomeric reaction product of a polyepoxide and an acrylo-butadiene copolymer having free carboxyl groups.

2. The elastomeric reaction product of a polyepoxide and a copolymer of butadiene and acrylic acid.

3. The elastomeric reaction product of a polyepoxide and a copolymer of butadiene and methacrylic acid.

4. The elastomeric reaction product of a polyepoxide and a copolymer of isoprene and acrylic acid.

5. A method of making a liquid acrylo-butadiene copolymer having free carboxyl groups which comprises copolymerizing 1,3-butadiene and a lower alkyl acrylate, heating the copolymer with an aqueous alkali metal hydroxide to replace the alkyl radicals of said copolymer with alkali metal, and adding acid to the resulting product to form said free carboxyl groups.

6. The method of making an elastomer which comprises copolymerizing a diene selected from the group consisting of butadiene and lower alkyl substituted butadienes with an acrylic compound selected from the group consisting of acrylic acid, lower alkyl substituted acrylic acids and the esters of said acids to form a liquid copolymer with a liquid polyepoxide, and heating the mixture to cause said polyepoxide to react with said copolymer to form a rubber-like solid.

7. A method according to claim 6 and wherein said diene is butadiene and said acrylic compound is methyl acrylate.

8. A method according to claim 6 and wherein said diene is butadiene and said acrylic acid compound is acrylic acid.

9. A method according to claim 6 and wherein said diene is b'utadiene and said acrylic compound is methacrylic acid.

10. A method according to claim 6 and wherein said diene is isoprene and said acrylic compound is acrylic acid.

11. The elastomeric reaction product of a liquid polyepoxide and a liquid acrylo-butadiene copolymer having free hydroxyl groups.

12. The elastomeric reaction product of a liquid polyepoxide and a liquid copolymer of butadiene and acrylic acid.

13. The elastomeric reaction product of a liquid polyepoxide and a liquid copolymer of butadiene and acrylic acid.

14. The elastomeric reaction product of a liquid epoxide and a liquid copolymer of isoprene and acrylic acid.

References Cited UNITED STATES PATENTS 2,947,338 4/1960 Reid et al. 26045.5ED 2,604,457 7/1952 Segall et al. 260455 2,604,464 10/1952 Segall et al. 26045.5 2,213,423 9/1940 Wiezevich 2604 2,740,702 4/1956 Mace M 52-0.5 2,682,461 6/1954 Hutchison 52-0.5 2,550,139 4/1951 Daly 2604S.5 2,600,024 6/1952 Romeyn 26045.5

OTHER REFERENCES Marvel et al., J. Polymer Sci.," 8, pp. 599605, 1952.

Marvel et al., Ind. Eng. Chem, 47, pp. 2221-23, 1955.

Potts et al., Dissert. Abstr., 14, pp. 1549- (1954).

LELAND A. SEBASTIAN, Primary Examiner US. Cl. X.R.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,563,966 Dated February 16, 197].

Inventor(s) It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Claim 6, line 7, before "with" insert --having reactiv carboxyl groups, mixing said copolymer Signed and sealed this 18th day of May 1971.

(SEAL) Attest:

EDWARD M.FLETGHER, JR. WILLIAM E. SCHUYLER, JR. Attesting Officer Commissioner of Patents 

1. AS A NEW COMPOSITION OF MATTER, THE ELASTOMERIC REACTION PRODUCT OF A POLYEPOXIDE AND AN ACRYLO-BUTADIENE COPOLYMER HAVING FREE CARBOXYL GROUPS.
 2. THE ELASTOMERIC REACTION PRODUCT OF A POLYEPOXIDE AND A COPOLYMER OF BUTADIENE AND ACRYLIC ACID.
 6. THE METHOD OF MAKING AN ELASTOMER WHICH COMPRISES COPOLYMERIZING A DIENE SELECTED FROM THE GROUP CONSISTING OF BUTADIENE AND LOWER ALKYL SUBSTITUTED BUTADIENES WITH A ACRYLIC COMPOUND SELECTED FROM THE GROUP CONSISTING OF ACRYLIC ACID, LOWER ALKYL SUBSTITUTED ACRYLIC ACIDS AND THE ESTERS OF SAID ACIDS TO FORM A LIQUID COPOLYMER WITH A LIQUID POLYEPOXIDE, AND HEATING THE MIXTURE TO CAUSE SAID POLYEPOXIDE TO REACT WITH SAID COPOLYMER TO FORM A RUBBER-LIKE SOLID. 